The present invention relates to ink jet printing, and in particular to an improved deflection electrode assembly for a continuous ink jet printer.
Continuous ink jet printers are well known in the field of industrial coding and marking, and are widely used for printing information, such as expiry dates, on various types of substrate passing the printer on production lines. As shown in FIG. 1, a jet of ink is broken up into a regular stream of uniform ink drops by an oscillating piezoelectric element. The drops then pass a charging electrode where the individual drops are charged to selected voltages. The drops then pass through a transverse electric field (deflection field) provided across a pair of deflection electrodes. Each drop is deflected by an amount that depends on its respective charge. If a drop is uncharged, it will pass through the deflection electrodes without deflection. Uncharged and slightly charged drops are collected in a catcher and returned to the ink supply for reuse. A drop following a trajectory that misses the catcher will impinge on the substrate at a point determined by the charge on the drop. Often, each charged drop is interspersed by a guard drop with substantially no charge to decrease electrostatic and aerodynamic interaction between charged drops. As the substrate moves past the printer, the placement of the drop on the substrate in the direction of motion of the substrate will have a component determined by the time at which the drop is released. The direction of motion of the substrate will hereinafter be referred to as the horizontal direction, and the direction perpendicular to this, in the plane of the substrate will hereinafter be referred to as the vertical direction. These directions are unrelated to the orientation of the substrate and printer in space. If the drops are deflected vertically, the placement of a drop in the vertical and horizontal direction is determined both by the charge on the drop and the position of the substrate.
As shown in FIG. 1, the print head of a continuous ink jet printer is often composed of a number of individual parts. For instance, the print head often contains a support frame, a low voltage electrode, a high voltage electrode, a resistor, an oscillating piezoelectric element, insulation, and a catcher. The high voltage electrode and low voltage electrode are generally separate and distinct pieces. The low voltage electrode is generally mounted to a support frame (not shown) for grounding. The high voltage electrode is typically connected in series with a resistor. Generally, the resistor limits discharge energy between the high voltage and low voltage electrodes under fault conditions.
One lead of the resistor is typically electrically connected to the high voltage electrode, and the other lead of the resistor is typically electrically connected to an external power circuit. The resistor is typically located within the print head, as shown in FIG. 1. As such, the environment of the resistor is typically filled with corrosive inks and cleaning solutions which may attack and compromise the functionality of the resistor. In order to protect the resistor from its harsh environment, the resistor is typically wrapped in sealing materials, which extend several inches from the ends of the resistor. The wrapping results in a stiff cable which is difficult to route and place among various tubes and lines during assembly and maintenance of the print head. Further, over time, the corrosive liquids can penetrate the wrappings, causing the resistor to fail. Accordingly, it is desirable to locate and shield the resistor from corrosive elements without wrapping the resistor in sealing materials during installation.
Also shown in FIG. 1, are the high voltage electrode and the low voltage electrode. The strength of the defection field, and thus proper operation of the ink jet, is a function of the spacing between the high voltage electrode and the low voltage electrode. If the gap between the electrodes is not optimized, the strength of the deflection field may be compromised, resulting in poor print quality and/or generating printer faults due to drops being deflected in undesirable locations.
The high voltage electrode and low voltage electrode are typically mounted separately to support structure within the printhead. Such mounting configuration typically requires a manual configuration of the gap between the high voltage electrode and the low voltage electrode. Manual configuration of the gap between the electrodes is prone to human error, thus exposing the printer to sub-optimal performance. Accordingly, it is desirable to have an assembly in which the spacing between the electrodes is predetermined, automatic, and optimized.
FIG. 1 also illustrates a dielectric insulator that may be used to prevent arching from the edges of a high voltage electrode to the ground electrode. Arcing is more probable at the edges of the high voltage electrode and where its distance to the ground electrode is minimal. This phenomenon has been reported in U.S. Pat. No. 6,848,774 for “Ink Jet Printer Deflection Electrode Assembly having a Dielectric Insulator.” Typically, the insulation is a loose piece, which is vulnerable to coming off during cleaning, or other operations. If the insulation does come off, the high voltage electrode may arc to the low voltage electrode, and the ink jet will operate improperly. Accordingly, it is desirable to have a special insulation which is robust during operation and maintenance.
Therefore, a need exists for a system and method for facilitating easier installation and improving robustness of a continuous ink jet printer. Such a system and method may protect a resistor from a corrosive environment without being wrapped. Moreover, such a system and method may easily optimize the space between the high voltage electrode and low voltage electrode. Furthermore, such a system and method may incorporate insulation so it is not easily detached.